Highly Sensitive Centrality Dependence of Elliptic Flow -- A Novel Signature of the Phase Transition in QCD

Elliptic flow of the hot, dense system which has been created in nucleus-nucleus collisions develops as a response to the initial azimuthal asymmetry of the reaction region. Here it is suggested that the magnitude of this response shows a ``kinky'' dependence on the centrality of collisions for which the system passes through a first-order or rapid transition between quark-gluon plasma and hadronic matter. We have studied the system Pb(158AGeV) on Pb employing a recent version of the transport theoretical approach RQMD and find the conjecture confirmed. The novel phase transition signature may be observable in present and forthcoming experiments at CERN-SPS and at RHIC, the BNL collider.